This invention relates to laminated safety glass assemblies and more particularly to such assemblies which contain polycarbonate or polyacrylate and polyvinyl butyral (PVB) sheets and which exhibit decreased delamination of the PVB on impact at very low temperatures.
In recent years sociologists and psychologists have started a movement toward prisons without bars. Thick layers of impact resistant polycarbonate initially used in such applications were determined to be inadequate since the polycarbonate could be burned or scratched or dissolved or fractured when heated or abused. Moreover, when exposed to the elements the polycarbonate turned yellow, lost its strength and was susceptable to crazing. Based on this early work a need was defined for a material for security applications which was strong, chemically impervious, and scratch and fracture resistant over a wide range of ambient conditions.
Thereafter laminates of one or more layers of glass with one or more layers of polycarbonate were proposed which in use were mounted with the glass on the outside exposed to the elements and the polycarbonate either forming the innermost surface or positioned inboard of a glass layer forming such innermost surface. However, with these laminates an interlayer between the glass and polycarbonate is necessary since polycarbonate will not adequately adhere to glass.
Thermoplastic polyurethane has been used to laminate polycarbonate to glass but the polyurethanes which have been used are costly. Commercially available grades of PVB sheet are not acceptable as an interlayer to adhere polycarbonate to glass because the plasticizer in the PVB crazes the polycarbonate. To solve this a special barrier coating, as disclosed in U.S. Pat. No. 4,243,719, was proposed between the polycarbonate and PVB to keep the plasticizer in the PVB away from the polycarbonate. The PVB is still necessary since the coating will not adhere to the glass and the PVB assists in absorbing energy on impact.
In the above-referenced commonly owned copending applications, PVB sheets are disclosed which are plasticized with an improved blend which does not craze polycarbonate or polyacrylate and wherein the sheets exhibit improved low temperature performance properties when present as a functional interlayer in a security glass laminate. However, when such laminates are exposed to very low temperatures on the order of minus 10.degree. C. the interface between the PVB and polycarbonate or polyacrylate is sufficiently weakened that on impact some delamination of the PVB from the polycarbonate or polyacrylate can be detected. Low temperature delamination is preferably avoided since it adversely affects panel integrity and consequently the degree of safety provided when used in a multilayered security panel.
A need, therefore, exists in the art for a precursor laminate containing plasticized PVB sheet and polycarbonate or polyacrylate which is usable with glass in laminated security glass applications which exhibits little or no delamination during impact at very low temperatures on the order of minus 10.degree. C.